The invention relates to a circuit arrangement for generating a useful direct voltage signal associated with the frequency of an alternating current signal, particularly for regulating the synchronism of an electric motor, the arrangement including a comparator circuit which converts two opposite polarity halfwaves of the alternating current signal into pulses whose edges trigger a measuring and control circuit which is able to generate direct voltages corresponding to the half-period durations of the alternating current signal measured from the respective zero passages.
Numerous circuits are known for converting the frequency of a sinusoidal signal voltage into a direct voltage that is proportional to that frequency. Frequently, the sample-and-hold principle is employed to furnish a frequency proportional direct voltage which has a sufficiently low residual ripple. After each trigger signal, a time proportional voltage is transmitted in bursts to a storage capacitor to replace the preceding measurement value. If only the ascending or descending zero passage of the signal voltage is selected as the trigger signal, it is possible to realize high repetition accuracy but only one trigger signal is obtained per full sinusoidal oscillation. However, trigger circuits are known already which furnish a trigger signal during each zero passage before or after the conversion of the sinusoidal voltage into a rectangular voltage (frequency doubling). These circuits are useful up to a certain accuracy. If, however, one desires to perform very precise angular velocity measurements, for example, to ensure or measure synchronism of a motor, two error sources must be compensated, one being caused by the offset voltage of the comparator and the other by periodic system specific harmonics in the tachogenerator.